1. Field of the Invention
The present invention relates to a digital signal processing device, a digital signal processing method, and a digital signal processing program which are preferably applied to a reproduction device to be mounted on a vehicle in which a digital signal processor (DSP) as a digital signal processing device is mounted.
2. Description of the Related Art
A hitherto known signal processing device samples an analog signal with the cycle of a predetermined sampling frequency using an analog-digital converter. In this way, the signal processing device generates a digital signal which is a data row of discrete data represented by digitalizing a value at an individual sampling point of an analog signal using the analog-digital converter. Then, the signal processing device sends the digital signal to a DSP and performs signal processing for the digital signal.
On the other hand, in the signal processing device, when a signal component of a frequency which is higher than the Nyquist frequency whose frequency is ½ of a sampling frequency with respect to an analog signal is included, for a digital signal generated by sampling the analog signal, a signal component of a frequency which is higher than the Nyquist frequency is returned with the Nyquist frequency being the border to generate a return noise which has its frequency converted to another frequency. The return noise generated in a digital signal is different from a signal component of a primary frequency included in the digital signal, and is a signal component which is hard to differentiate from a signal component of the primary frequency.
In the signal processing device, when performing signal processing for a digital signal in which a return noise is generated in a DSP, the return noise is wrongly detected as a signal component of a primary frequency of a digital signal, and, as a result, there is a fear of malfunction. Accordingly, the signal processing device controls the frequency band of the analog signal to the Nyquist frequency or lower by making the analog signal pass through a lowpass filter, and performs the sampling to generate a digital signal. In this way, the signal processing device prevents a return noise from occurring in a digital signal (refer to, e.g., Jpn. Pat. Appln. Laid-Open Publication No. 2000-211512 [page 2, FIG. 2]).